Author: Tim Hunt
MDL-63260 qtype_random: orphaned random questions should be deleted
This fix is based on some original code written by Bo Pierce.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=a637d7782637eaa879e8034878fdf4f49484c79c
http://github.com/moodle/moodle/commit/a637d7782637eaa879e8034878fdf4f49484c79c
+ 3.6 en [taskunusedrandomscleanup,qtype_random]